LiPos are nominally 3.7V per cell, so the closest would be a 2S battery, but this gives 7.4V nominal voltage and up to 8.4V when fully charged, which may well be too much if the circuit is only expecting 6V.

I would use a 3S (11.1V) cell as the power source and a battery eliminator (BEC) unit to create a stable 6 volt supply.